1. Field of the Invention
The invention is related to a porcine circovirus type 2 (PCV2) subunit vaccine, particularly to a PCV2 subunit vaccine having a peptide of PCV2 open reading frame (ORF2) that can be abundantly expressed as an antigen and an additionally proper carrier or adjuvant.
2. Description of the Prior Art
It is known that porcine circovirus type 2 (PCV2) is related to postweaning multisystemic wasting syndrome (PMWS) and porcine dermatitis and nephropathy syndrome (PDNS). PMWS was first found in pigs in Canada in 1991 and has been reported subsequently around the world. The syndrome has caused a huge loss in swine production industry worldwide. Main symptoms of PMWS include progressive weight loss, tachypnea, dyspnea, jaundice, et cetera. Visibly pathological changes in tissue include lymphocytic and granulomatous infiltrate, lymphadenopathy, lymphocytic and granulomatous hepatitis, and nephritis.
Porcine circovirus (PCV) was first recognized in a pig kidney cell line (PK-15, ATCC CCL33) in 1982. Although the porcine circovirus can continuously contaminate PK-15 cells, the virus does not cause cytopathic effect (CPE) in the contaminated PK-15 cells. Even though the porcine circovirus can infect pigs, it does not cause lesions in the infected pigs. The virus is named porcine circovirus type 1 (PCV1). PCV1 is an icosahedron, single-stranded DNA virus with a circular genome of 1,759 bp. The PK-15-derived PCV was classified in the Circoviridae family in 1995.
The PK-15-derived PCV1 is considered apathogenic. In contrast, the virus mutation isolated from pigs with PMWS in 1997 is pathogenic and is named porcine circovirus type 2 (PCV2). Postweaning Multisystemic Wasting Syndrome (PMWS) is a highly contagious pig disease. It mainly infects pregnant sows and their piglets and seriously affects health of pigs.
PCV2 is a single-stranded, circular DNA virus with a diameter of 17 nm, and its genome size is 1.76 kb. Genomic analysis with software shows a total of 11 open reading frames (ORFs) transcribed in the clockwise and counterclockwise directions. Among the 11 ORFs, ORF1 and ORF2 are probably the most important genes. ORF1 gene encodes Rep and Rep′ proteins, which are related to virus replication. It is known that ORF2 gene encodes immunogenic structure capsid protein of PCV2, which is used to induce immune response in organisms.
Inactivated PCV2 vaccine is the most common commercially available PCV2 vaccine. However, developing inactivated vaccine requires cell lines to be free of contaminant, and the possibility of incomplete inactivation of the virus by chemicals is the most significant disadvantage of inactivated vaccine. Another disadvantage is that the antigenic structures of the virus may be altered by chemical treatment, leading to failure to induce sufficient immune response to eliminate the virus and failure to protect pigs from infection of the disease. Therefore, developing inactivated vaccine can be difficult and costly, and vaccine safety may be a concern.
Unlike inactivated vaccine, in which the whole virus is the vaccine antigen, subunit vaccine uses a part of proteins from the pathogen as antigen protein, and the antigen protein is inoculated into animals or humans to induce immunity. Subunit vaccine can be prepared by cloning genes encoding antigen proteins from pathogens and then producing large amounts of the antigen proteins by genetic engineering. Safety is the most significant advantage of subunit vaccine because it uses parts of a pathogen, instead of a whole pathogen, to inoculate pigs without the issue of incomplete inactivation. Conventional PCV2 subunit vaccine uses PCV2 ORF2 protein as the antigen protein; however, the protein expression level of full-length PCV2 ORF2 protein in prokaryotic expression systems is quite low and does not meet the requirements of vaccine production. Therefore, developing antigen fragments of PCV2 ORF2 that can be highly expressed in biological expression systems is helpful to commercial application of PCV2 subunit vaccine.